Transmutation of long-lived fission products (LLFPs: $^{79}$Se, $^{93}$Zr, $^{99}$Tc, $^{107}$Pd, $^{129}$I, and $^{135}$Cs) into short-lived or non-radioactive nuclides by fast reactors without isotope separation nor even-odd separation has been proposed as a solution to the problem of radioactive wastes disposal[1,2]. Despite investigation of many methods, such transmutation remains technologically difficult. To establish an effective and efficient transmutation system, we propose a novel neutron moderator material, yttrium deuteride (YD$_2$), to soften the neutron spectrum leaking from the reactor core as well as YH$_2$. Special care has been taken to avoid thermal spike/peaking in reactor heat distribution. Neutron energy spectra and effective halflives of LLFPs, transmutation rates, and support ratios were evaluated with the continuous-energy Monte Carlo code MVP-II/MVP-BURN and the JENDL–4.0 cross section library. The effective half-lives of the 6 LLFPs will be drastically decreased from the order of 10$^6$ to 10$^2$ years, and the support ratios exceed 1.0 for all six LLFPs. This successful development and implementation of a transmutation system for LLFPs without isotope separation contributes to an ability of fast reactors to reduce radioactive waste by consuming their own LLFPs.

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This study comprises the results of “Research and development of an innovative transmutation system of LLFP by fast reactors” (18K03642) entrusted to the Tokyo Institute of Technology by the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT).

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[1] S.Chiba, T.Wakabayshi, Y.Tachi, N.Takaki, A.Terashima, S.Okumura and T.Yoshida, Scientific Reports 7, 13961(2017).

[2] T.Wakabayashi, M.Takahashi, S.Chiba, N.Takaki, Y.Tachi and Y.Tahara, Nucl. Eng. Design 352(2019)110208.